Distance measuring system



Feb. 7, 1950 E. E. TURNER, JR

DISTANCE MEASURING SYSTEM Filed Nov. 29, 1947 TRIGGER PU LS E GENERATORTO SAW-TOOTH QGENERIATOR INVENTOR.

TO VACUUM EDwlN TURNER JR.

TUBE VOLTMETER BY 694 HIS A OREY Zatented Feb. 1?, l?

ATES PATENT OFFICE Edwin E. Turner, Jr., West Roxbury, Mass., as-

signor to Raytheon Manufacturing Company, a corporation of DelawareApplication November, 1947, Serial No. 788,811

6 Claims.

The present invention relates generally to radio echo detecting andranging systems, and more particularly to a system having very highranging accuracy and very low minimum range. The invention isparticularly suited for use in altimeter and collision warning systemsin aircraft.

An absolute radio altimeter which will constantly indicate with a highdegree of accuracy the height above ground, even to low values of theorder of ten feet, of a fast-moving aircraft at all times and with theutmost simplicity has been sought after for some time. Such a systemrequires the employment of extremely short transmitted pulses, andconsequently the band width of the :eceiver-system must be extremelygreat. While radio echo altimeter systems have heretofore been devised,they have usually been lacking in high ranging accuracy and low minimumrange, and have been heavy and complex in structure. The presentinvention provides a system which possesses low minimum range with highranging accuracy, and may be made light in weight and relatively simplein construction, and which provides the desired range information as anindication on a conventional direct-reading galvanometer type meterwhich may be located where desired. As additional features, the metermay be adjusted to read range or height accurately from within thesystem, providing a reliable and simple performance check.

The invention envisions the employment of a very simple spark-typetransmitter which provides exceedingly short pulses of electromagneticradiation, of the order of 0.01 microsecond.

These pulses are generated and radiated through a single unitaryinstrumentality. There is provided also an equally simple receiving,detecting, and amplifying system employing a separate antenna whichincorporates a detector within itself, and as an amplifier only oneelectron tube of the electron multiplier type. The receiver system isnot tuned and hence is responsive to an exceptionally wide band offrequencies. In addition it requires little or no servicing. Rangeinformation is constantly provided on .a meter through a, memory devicewhich operates at low voltages and is actuated only toreeord changes in.the condition being observed. The memory device is arranged to performaccurately, and is not adversely afiected by the indicating deviceconnected thereto.

The invention is described below in connection with the drawing, whichshows schematically in Fig. 1 an apparatus that may be used as anabsolute altimeter or as a collision warning device,

and in Fig. 2 a modification to provide for calibration of the meter.

In the system shown in Fig. 1, pulsed electromagnetic radiation isgenerated and propagated into space by a pulse generator-transmitter l0,shown generally in a dotted-line box. Pulse generator-transmitterssuitable for the purposes of the present invention are shown in variousforms in Patents No. 2,407,245; 2,408,405; 2,408,406; and 2,417,052. Thepulse generator-transmitter l0 shown in Fig. 1 is generally like Fig. 2of Patent No. 2,417,052, and will not be further described except toidentify its principal parts, which are a spark gap capacitor members l2thereto connected, dipole radiators I3, a reflector H, and a. hightension step-up transformer IS. A storage capacitor l6 provides theenergy for spark discharges. A battery i'l provides current for chargingthe capacitor It, the charging path being completed through a protectiveresistor I8 to ground, and the primary winding 2| of the transformer l5.A normally non-conductive electronic switch tube l9, which is preferablya gaseous discharge tube, and may desirably be of the hydrogen thyratrontype, discharges the capacitor IE to ground when rendered conductive, toprovide a current surge in the primary winding 2| of the transformer I5.This surge provides a high voltage in the secondary winding 22, and apulse of electromagnetic energy is generated, as described in the abovementioned patents. The resistor It prevents short-circulting of thebattery l'l during conductivity of the tube It.

A trigger pulse generator 23 provides a very short positive voltagepulse 24 which is fed to the switch tube It! and to a saw-tooth voltagegenerator 25 in parallel. The positive pulse 23 is applied to thecontrol grid of the switch tube l9 through a coupling capacitor 26. Thisgrid is biased negatively with respect to the cathode by means of abattery 21, which .is connected between the grid and the cathode througha current limiting resistor 28. The positive charge stored in thecapacitor 26 after a positive pulse 24 has been applied to the gridleaks oil through the resistor 28, ground, and the ground connectionwire 29 of the pulse generator 23, so that the tube i9 soon againbecomes non-conductive in preparation for the next trigger pulse. Thedischarge path of the storage capacitor i6 is through the tube IS, theprimary winding 84 of a transformer 83 (the purpose of which will beexplained below), ground, and the primary winding 2| of the pulsetransformer I5. The impedance of this path is very low, so that thecapacitor I5 discharges very quickly and the tube I9 is thereforequickly exorder of 300,000 pulses per second, which are useiul in shortrange systems for altimeter purposes.

The saw-tooth voltage generator 25 is 01' a conventional form, andcomprises an electron tube 30, which may be of the gaseous type, acharging capacitor 3| wherein a substantially linearly increasingvoltage is periodically generated, and a charging resistor 32. Thecharging cap citor 3| and charging resistor 32 are connected in seriesacross a source of unidirectional voltage, B to ground, and the tube 30is connected at its anode and cathode across the capacitor 8|. Aresistor I3 is provided in the cathode circuit; through which thecharging current flows and biases the cathode positively with respect toground. The charging capacitor Si is charged only to a small fraction ofthe available B voltage at any time, so that the voltage across thecapacitor increases substantially linearly with time during eachcharging operation. The control electrode 80 01 the tube 30 is connectedto ground through a resistor 84, and to the cathode through theresistors 33 and 34 in series, and is thus biased neg atively withrespect to the cathode. The bias pro vided is sumcient to maintain thetube 30 nonconductive for the range of anode-cathode potentials that isused. The positive pulse 24 is applied to the control electrode 38through a coupling capacitor 35, and renders the tube 38 conductive atabout the same time that the generator-transmitter switch tube I9 isfired. The positive voltage that is thus applied to the controlelectrode 86 leaks oil through the grid resistor 86, ground, and thetrigger pulse ground wire connection 29 and, due to the low anodecathodevoltage that exists when the charging capacitor SI is discharged throughthe tube, the tube 30 is very quickly rendered non-conductive again.Recharging or the capacitor 3| provides the positive cathode (ornegative grid) bias through the cathode resistor 33 to maintain the tubenon-conductive until the next succeeding trigger pulse 24. 7

At the termination of the conductive period of the tube 30, the nextsucceeding linear voltage sweep commences, almost simultaneously withthe generation and transmission of a pulse of radio frequency energy bythe generator-transmitter ill. The discharge time of the saw-toothgenerator capacitor 3| through the tube 30 may introduce some delay inthe commencement oi the sweep voltage. To avoid this delay and start thesweep voltage exactly when the energy pulse is transmitted, a phaseshifter 3! may be interposed between the trigger pulse generator 23 andthe switch tube 89 if desired, so that the trigger pulse 24 fed to theswitch tube will arrive there a small time after the saw-tooth voltagegenerator tube 30 is fired. The saw-tooth voltage generated in thesawtooth generator capacitor 31 is applied across a measuring capacitor40 through a normally-open electronic switch 4| and ground. Thenormallyopen switch is controlled by a radio receiver, as will bedescribed below.

The radio receiver system has an antenna 42 which may include as itselements a dipole ll and a parabolic reflector 44. A unidirectionalconductor 45, for example a crystal, is connected directly between theelements of the dipole 48. as a detector, so that pulsed unidirectionalcurrent only appears in the antenna connection cable II when echoes ofthe transmitted energy are received in the receiver antenna 42. Toprotect the crystal ll from being burned out during the generation andtransmission of energy pulses, the generator-transmitter II and thereceiver antenna 42 may be located at opposite ends of the wing oi theaircraft.

'.Unidirectional current pulses from the receiving antenna 42 arebrought to a wide-band ampliner which comprises an electron-multipliertube ll having a photoemissive cathode ii, a control electrode I2, aplurality oi secondary electron emitters 58, an anode 54, and a lightsource 8!. The light source SI is energized by a battery SI, andarranged to illuminate the cathode 5| which emits electrons. A battery51 provides positive potentials for the secondary electron emitters BIand the anode H. The negative terminal of the battery 51 and the cathodeii are grounded, while the positive terminal 01' the battery isconnected tothe anode I through a plate load resistor 84. Each of thesecondary electron emitters BI is connected to an individualintermediate point on the battery I! through a current limiting resistor88. The secondary electron emitters 53 are each made increasinglypositive with relation to the cathode ii, the most positive secondaryelectron emitter being the one furthest from the cathode II, or nearestto the anode N. Electrons emitted from the cathode are thus attracted tothe secondary emitter 53 nearest to the cathode, where by impact theycause the emission of an increased number of secondary electrons. Thesecondary electrons are further attracted to the next furthest removedsecondary emitter, where the process is repeated, until the electronstream that arrives at the anode 54 is vastly multiplied in density bycomparison with the electron stream emitted by the cathode.

The particular electron multiplier tube employed may have aphotoemissive cathode and a light source, as shown and described hereinor, if desired, a thermionic cathode may be employed. Either kind ofcathode can furnish electrons by random emission. However, thethermionic cathode requires heat and consequently eventually causescontamination of the secondary emitter surfaces. In the practicalconstructions of photoemissive electron multipliers that are available,the light source is either enclosed, or the light is brought to thecathode by a devious path by way of a mirror, so that no contaminationof the secondary emitter surfaces occurs. Due to these advantageouscharacteristics, the photoeinissive electron multiplier provides a veryhighly satisfactory wide band amplifier, having a high signal-to-noiseratio.

The flow of electrons from the cathode 5| is controlled by a controlgrid 52, interposed between the cathode and the nearest secondaryemitter 53. The control grid 52 is normally biased negatively withrespect to the cathode 5| by means of a battery 59 connected between thecathode and the control grid through a resistor 0|. The electron streamis thus held to a certain low, steady-state value, or, if desired, cutoil altogether. The unidirectional signals from the antenna 42 areapplied across the grid resistor Bl through a capacitor 62 in such afashion as to drive the control grid 52 in a positive direction withrelation to the cathode 5|. A resistor 03 of high value is connectedacross the unidirectional conductor 45, and'the charge thus given to thecontrol grid 52 leaks ofl through this resistor, through a circuitincluding the bias battery 59 and the'grid resistor'ili The timeconstanto f this circuit is preferably chosen to be very short. The gridcontrol is effective where the electron density is low,- and thereafterampliflcation takesplaces as a characteristic of the tube Eill itself.creases in density when an echo pulse is re I ceivedin' the antenna 42,and the resultant currentjflow in the'plate load resistor 64 causes aThe electron stream thus involtage drop-at the anode 54 which isemployed to close the-normally open switch ll to place a charge on themeasuring capacitor 40 in a manner that .will now be described.

The normally open switch 4i comprises a pair oftriode sets, the firstset comprising a first q anode 65, first cathode 66, and'first controlelectrode 61, and the second set comprising a second anode H, secondcathode I2, and second control electrode I3. 7 The first anode 65 andthe second cathode 12 are connected together and to the output of thesaw -tooth generator 25. The first cathode 66 and second anode Hare'connected together and to one side of the measuring capacitor 40.The other side "of the' measuring capacitor is grounded. The firstcontrol electrade 61 is connected to the first cathode 66 through thesecondary winding 68 of a first pulse transformer l8 and a first biasbattery 69 in series. 7

nected to the second cathode 12 through the I The second controlelectrode 13 is conform of a unidirectional output pulse, and isconnected through a coupling capacitor 16 to the primary windings 8i and82 of the first and second pulse transformers i8 and 19in series. Theconnection is such that, when there is a receiver output signal, bothcontrol electrodes 81' and 73 are driven suiliciently positive withrelav tionto their respective cathodes 66 and I2 to overcome the biasfurnished by the batteries 69 I 6 instant chargedmorepositively t huring capacitorl l'the first anode cathode it become conductive; 01

tooth generator capacitor 3| is at charged only to a value that is neg:spect tothe potential of the n pacitor All, the second anode II and odel2 become conductive. In eitl measuring capacitor 40 is momenta] to thesaw-tooth generator capacit suines the same charge, or more samepotential level above ground. of the switch 6i is momentary onl: pulsetransformers unblock their n trol electrodes only when there is. a agechange in the plate load resis sudden change is insensitive to th thepulse that follows. The lock-cu former is connected in the signalcircuit in a fashion to oppose the two signal pulse transformers 8| aeffective to prevent unblocking o electrodes 61 and 73 during the grtransmission of an energy pulse. rent that may be drawn by the coni 61and i3 is confined to the respect: cathode circuit, and does not effectthe measuring capacitor 6 0. Such if it does occur, may be minimizedsion of grid circuit resistors (not s respective grid-cathodeconnection:

The potential that is present at a! measuring capacitor M is indicatedtube voltmeter circuit comprising tubes ti and 92. The first tube 9| atits anode 93 directly to a sourc1 tional voltage B, which is preferatand 15 respectively. The transformer 83 is employed as a lock-out pulsetransformer and has its primary winding 84 connected in series in theanode-cathode circuit of the generator-transbattery 51, and the outputresistor 64 in which the output signal appears. Any charge remaining onthe coupling capacitor 16 after the trans.- mission of a pulsetherethrough is dissipated through the signal transmitting circuit. Thetime constant of this circuit is preferably very short.

The output pulse thus furnished to the normally open switch ii overcomesthe negative bias of each triode section. The triode sections arereversely connected in parallel in the circuit to the measuringcapacitor 40, so that one or the other triode section conducts currentdepending on which of the two capacitors 3i and 40 has the iii higherpotential at the time when the output pulse is impressed on the switch4|. Thus if the saw-tooth generator capacitor 3i is at that lated, andat its cathode 94 to gr a cathode resistor 95. The contro is connectedto the switch side of t capacitor 36. The second tube 92 at its anode 91to the B supply, and 98 to ground through a second ca 99. A potentialdivider I0! is com ly from B to ground, and the secom electrode 062 isconnected to the po through a variable tap I03. A met may be a sensitivegalvanometer, a resistor Hi5 are connected in series two cathodes 9 4iand 98.

The first tube 9i of the vacuum t is operated as a cathode follower,substantially infinite impedance, draws grid current. Th cathode lowsthe grid potential very accurz charge on the measuring capacitor 1sipated through the tube. The $1 operated as a voltage divider, and the.adjustable tap Hi3, adjusts thv the second tube cathode 98 to b that ofthefirst tube cathode 94 W1 W4 is expected to read zero. T tap m3 isthus-a minimum re: mentl -The variable resistor I05 amount of meterdeflection per voli adjusts the meter to read a desire( value; These twoadjustments a calibration adjustments 'of the sy:

.will be shown, can be made within In the system of the invention dethemeasuring capacitor 40 fui memory device with respect to distance toa single target that is l: ously observed, for example thee a movingaircraft. Since the resi 2,400,000 7 Le saw-tooth generator, or andrestore normal operation of the measuring id the measuring capacitorcapacitor switch I.

.y a few signals need be re- Many modifications of the above describedemr to charge the measuring bodiment may be made within the scope of thepotential level as the sweep invention to accommodate a particularemployr if the range, or in this ment thereof. For example, the meterI04 may creasing, each received sig- Y be replaced by any one of manyforms of threshrender the switch dl conhold alarms known to the art, andthe invenrst anode 65 and first cathtion can then serve to light a lampor sound 3. oltage increases with tim 10 bell when the altitude of theaircraft becomes the altitude is decreasing, dangerously low or acollision becomes imminent. d ec nd cathode 12 will In certain of itsaspects, for example the system during each observation for controllingthe measuring capacitor switch 4| ring capacitor do will then for themeasuring capacitor W, the invention may tively than the sweep ca- 15find employment in other types of echo ranging no change in the altitudeequipment.

ceived echoes, neither tri- Having now described my invention, I claim:ch 4! will become conduc 1. In a system for distance measurement bycontrol electrodes 67 and the time of travel method, in combination a ydisposed to permit conspark-gap transmitter for generating and proper--anodes and cathodes will eating a pulse of oscillatory energy; and a re-)tential, and no electrons ceiving circuit including a dipole antennahaving :1- triode section As mena unidirectional conductor connectedacross the rrent should occur, it is 10- output thereof and anelectron-multiplier type rive grid-cathode circuits, amplifier connectedto said output. charge on the measuring 2. In a system for distancemeasurement by acuum tube voltmeter has the time of travel method, incombination: a ut impedance, the memspark-gap transmitter for generatingand propaag capacitor it is very gating a pulse of oscillatory-energy;and a re- =itor holds its charge for a ceiving circuit including adipole antenna having lSBIiCB of signals that alter a unidirectionalconductor connected across the on ofthe system. Alteraoutput thereof andan electron-multiplier type he measuring capacitor is amplifier with aphotoemissive cathode and a 1, so that there is no delay source ofillumination therefor connected to said es in terrain contour, oroutput.

)rizontally directed meas- 3. In a system for distance measurement by asmentioned above, the the time of travel method, in combination: a 1gcapacitor it has little spark-gap transmitter for generating andpropasuclclen changes in range gating a pulse of oscillatory energy; areceiving .ccurate. The meter I04 40 circuit including a dipole antennahaving auniide directly. directional conductor connected across the out-;th is known in terms of put thereof and an electron-multiplier type am-:he sweep is synchronized plifier connected'to said output; means forgenllse, the meter i5 2 can be erating a timing wave; a substantiallyisolated system by means of the charge storage device; means responsiveto the Pig. 2. To the measuring output of said amplifier for momentarilyapplye added two single-pole ing said timing wave to said storage devicein reiii and H2. The first sponse to the receipt of an echo; and meansfor rum, or zero range, adindicating the charge stored in said storagedeally this switch connects vice. :trode 13 to the second 4. In a systemfor distance measurement by Fig. l, but for calibration the time oftravel method, in combination: a annected directly to the spark-gaptransmitter for generating and propasecond triode section is gating apulse of oscillatory energy; a receiving ve over the whole sweep circuitincluding a dipole antenna having a uni- L generator 25, and thedirectional conductor connected across the outassumes the lowest sweepput thereof and an electron-multiplier type amevacuum tube voltmeter,plifier connected to said output; a timing ca-. aent tap I03 is thenadpacitor; means to charge said capacitor to a voltdifference betweenthe age that varies in-accordance with the time" s zero, and the meterI04 elapsed after transmission of said pulse; a sub- 3 switch Iii is nowrestantially isolated charge storage capacitor; a osition, and thesecond normally open switch connected between the two ;o the calibrateposition. capacitors and operatively connected to theoutiking the firsttriode secput of said amplifier for momentary closure upon the firstanode and the receipt of an-echo; and means for indicating m entiresweep voltage the charge stored in said storage capacitor.

lg capacitor til assumes 5. In a system for distance measurementby eavailable. In the vacthe time of travel method, in combination: a

maximum adjustment 7 spark-gap transmitter for generating andpropaliusted until the meter gating a pulse of oscillatory energy; areceiving "e to which the full sweep circuit including a dipole antennahaving a unihe second switch H2 is directional conductor connectedacross the outmal position to connect putthereof and anelectron-multiplier type am- :le 6! to its cathode 66 7 plifierconnected to said output; means for generating a timing wave; asubstantially isolated charge storage device; means responsive to theoutput of said amplifier for momentarily apply-- in'g said timing waveto said storage device in response to the receipt of an echo; and avacuum tube voltmeter having substantially infinite input impedance forindicating the charge stored in said storage device.

6. In a system for distance measurement by the time of travel method, incombination; a spark-gap transmitter for generating and propagating apulse of oscillatory energy; a receiving circuit including a dipoleantenna having a unidirectional conductor connected across the outputthereof and an electron-multiplier type amplifier connected to saidoutput; means for generating a timing wave; a substantially isolatedcharge storage device; means for applying said timing" wave to saidstorage device including a pair of reversely disposed, parallelconnected triode sections connected between the timing wave generatingmeans and the storage device, the grid of each triode being biased to acut-oil potential by a bias circuit connected between the 10 grid andcathode; an inductive connection between the output of said amplifierand said grids arranged to overcome the bias of both triodes in responseto the receipt of an echo; and means for indicating the charge stored insaid storage device.

EDWIN E. TURNER, Ja.

REFERENCES CITED The following references are of record in the fileofthis patent;

UNITED STATES PATENTS

